Looks like I found my answer.
This is from this site: http://rmcavoy.freeshell.org/6to8.html
These years used non-interlocking engine mounts (aftermarket interlocking replacements are available). The frame halves of these mounts (perches) are made of steel and the engine halves are made of steel/rubber. Both halves of these mounts are specific to the engine (L6, small block V8, or big block V8). Therefore, both halves need to be replaced during an engine swap.
So, since the 6 cylinder engine I removed had steel chassis mounts and steel/rubber engine mounts, I would have to replace my chassis mounts. Which means I have to cut those existing mounts out.